1. Field of the Invention
The present invention relates to a polypropylene resin composition superior in coating properties. More particularly, the present invention relates to a polypropylene resin composition applicable for the products which needs painting, such as automobile bumpers.
2. Description of the Prior Art
Generally, the non-polarity of polypropylene makes it difficult to apply printing, bonding, and coating or painting on polypropylene-based products. To improve the coating properties of polypropylene-based products, much effort has been made, including the formation of a stable coating layer on the polypropylene-based products. For example, the adhesion between a polypropylene-based product and a paint coat can be improved by pretreatments, such as etching with halogenated hydrocarbons and primers. However, because the halogenated hydrocarbons of which 1,1,1-trichloroethane (TCE) is representative, are found to cause depletion of the ozone layer, their use is prohibited or severely regulated. The pretreatment with primers has an economical disadvantage of giving rise to an increase in the cost of the end product because the primers are usually expensive.
In addition, the polypropylene-based products of which surfaces are treated with chromic acid, flame, corona or plasma, are better printed or painted. However, these surface treatment techniques are still unsatisfactory in environmental and economical aspects.
With the aim of improving the coating properties of polypropylene-based products was disclosed a group of methods, which are based on physical and/or chemical modification, such as introduction of and/or blending with polar groups.
U.S. Pat. No. 4,946,896 discloses a polypropylene composition comprising 20 to 80% of polypropylene, 5 to 38% of a copolymer consisting of ethylene, alkyl acrylate or methacrylate and unsaturated carboxylic acid anhydride, and 5 to 70% of ethylene-polypropylene rubber. In this composition, the ethylenic copolymer plays a role as a polarity source, but is so poor in compatibility with polypropylene that deteriorated physical properties and non- uniform coating properties are attained.
U.S. Pat. No. 5,424,362 discloses a composition comprising 30 to 70% of polypropylene or graft-modified polypropylene, 0 to 40% of an ethylenic polymer or graft-modified ethylenic polymer, and 0 to 50% of an ethylene-unsaturated carboxylic acid copolymer. Owing to high molecular weights, the graft-modified polyolefins cannot be distributed effectively over the surface of injection-molded products. Thus, a large quantity of the modified polyolefin is needed to provide satisfactory coating properties to the products.
U.S. Pat. No. 4,997,720 discloses a composition comprising 1 to 50% polypropylene, 3 to 50% of an ethylene-alpha olefin copolymer, 2 to 25% of a copolymer of ethylene and unsaturated carboxylic acid, 2 to 20% of an epoxy or phenoxy resin, and optionally 50% or less of inorganic fillers. The copolymer of ethylene and unsaturated carboxylic acid is too high in molecular weight and poor in the compatibility with polypropylene to provide satisfactory coating properties.
U.S. Pat. No. 5,486,561 discloses a composition comprising 50 to 90% of polypropylene, 10 to 50% of ethylene-alpha olefin copolymer, and 0.1 to 15 weight parts of a polyolefin modified with a functional group, based on 100 weight parts of the polypropylene and the copolymer in total. However, the products made of this composition are required to be treated with primers prior to painting.